BTG, the British patent licensing company, is suing a group of American retailers including Amazon.com, the largest online retailer in the world, for allegedly infringing its rights over a technology to track customers' use of the internet.

The company's lawsuit, filed in a Delaware court, claims the retailers are using a technique it has already patented to monitor when customers move from one website to another. The technology is important because websites selling goods will normally pay a fee to other sites thatdirect traffic their way.

BTG is claiming an undisclosed amount of damages from the group, which also includes BarnesandNoble.com, the electronic version of America's pervasive bookshop chain. BTG buys patent rights to new technologies and licenses them to manufacturers. It also sets up its own companies to develop technology and is best known for its subsidiary Provensis, which is testing a revolutionary new varicose vein treatment. The treatment ran into trouble at the end of last year when US regulators suspended its tests.

The company, originally set up by the Government to protect and patent the country's inventions, has a history of taking on opponents much larger than itself to protect its wide-ranging intellectual property rights.

Earlier this month BTG sued Microsoft and Apple for including patented technology in their operating systems that allows computer users to obtain software updates over the internet.

In a statement about the latest law suit, BTG said: "The suit asks for unspecified damages for past infringing activity and an injunction against future use of the technology." It said it had tried to reach an agreement but had failed. Fighting the case in court could take three years, making it possible that the parties will yet reach an agreement over the technology.

Ian Harvey, BTG's chief executive, said the patents are "fundamental to the tracking of users for online marketing programmes", adding that the technology's commercial potential is "significant". BTG's shares moved ahead 4p to 140p.